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>25.3. Failover</A
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><P
>    If the primary server fails then the standby server should begin
    failover procedures.
   </P
><P
>    If the standby server fails then no failover need take place. If the
    standby server can be restarted, even some time later, then the recovery
    process can also be restarted immediately, taking advantage of
    restartable recovery. If the standby server cannot be restarted, then a
    full new standby server instance should be created.
   </P
><P
>    If the primary server fails and the standby server becomes the
    new primary, and then the old primary restarts, you must have
    a mechanism for informing the old primary that it is no longer the primary. This is
    sometimes known as <ACRONYM
CLASS="ACRONYM"
>STONITH</ACRONYM
> (Shoot The Other Node In The Head), which is
    necessary to avoid situations where both systems think they are the
    primary, which will lead to confusion and ultimately data loss.
   </P
><P
>    Many failover systems use just two systems, the primary and the standby,
    connected by some kind of heartbeat mechanism to continually verify the
    connectivity between the two and the viability of the primary. It is
    also possible to use a third system (called a witness server) to prevent
    some cases of inappropriate failover, but the additional complexity
    might not be worthwhile unless it is set up with sufficient care and
    rigorous testing.
   </P
><P
>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does not provide the system
    software required to identify a failure on the primary and notify
    the standby database server.  Many such tools exist and are well
    integrated with the operating system facilities required for
    successful failover, such as IP address migration.
   </P
><P
>    Once failover to the standby occurs, there is only a
    single server in operation. This is known as a degenerate state.
    The former standby is now the primary, but the former primary is down
    and might stay down.  To return to normal operation, a standby server
    must be recreated,
    either on the former primary system when it comes up, or on a third,
    possibly new, system. Once complete, the primary and standby can be
    considered to have switched roles. Some people choose to use a third
    server to provide backup for the new primary until the new standby
    server is recreated,
    though clearly this complicates the system configuration and
    operational processes.
   </P
><P
>    So, switching from primary to standby server can be fast but requires
    some time to re-prepare the failover cluster. Regular switching from
    primary to standby is useful, since it allows regular downtime on
    each system for maintenance. This also serves as a test of the
    failover mechanism to ensure that it will really work when you need it.
    Written administration procedures are advised.
   </P
><P
>    To trigger failover of a log-shipping standby server,
    run <TT
CLASS="COMMAND"
>pg_ctl promote</TT
> or create a trigger
    file with the file name and path specified by the <TT
CLASS="VARNAME"
>trigger_file</TT
>
    setting in <TT
CLASS="FILENAME"
>recovery.conf</TT
>. If you're planning to use
    <TT
CLASS="COMMAND"
>pg_ctl promote</TT
> to fail over, <TT
CLASS="VARNAME"
>trigger_file</TT
> is
    not required. If you're setting up the reporting servers that are
    only used to offload read-only queries from the primary, not for high
    availability purposes, you don't need to promote it.
   </P
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